A very simple class of measurements based upon interactions of pairs of electrons called near zero field magnetoresistance (NZFMR) spectroscopy has much of the analytical power of much more complex electron paramagnetic techniques in identifying the physical and chemical nature of point defects which play important roles in the operation of a wide variety of solid state devices. We show that this very simple measurement can provide information about hyperfine interactions with dominating defects in technologically important devices.
Individual magnetic impurities or small collections of magnetic impurities in III-V semiconductors can be identified via scanning tunneling microscopy (STM) [1,2], their exchange interaction can be measured [3], and they can have remarkably long spin coherence times [4]. Spin-1/2 impurities are able to be addressed individually and the eigenstates tailored allowing the construction of engineered spin networks [5]. We describe an approach to explore the coherent spin dynamics of a spin-1/2 defect coupled to an additional spin-1/2 defect via exchange interaction with a spin-polarized STM contact through low-field magnetoresistance. The inherent anisotropy [2,3,5,6] in conjunction with the applied magnetic field should allow one to describe a single spin Hanle curve. In addition, measurements of the spin coherence time and the local hyperfine interaction should be feasible. This analysis is then used to guide the examination of coherent spin-dynamics involving coupled Mn-hole complexes in III-V semiconductors.
[1] J. M. Tang and M. E. Flatté, Phys. Rev. Lett. 92, 047201 (2004).
[2] A. M. Yakunin et al., Phys. Rev. Lett. 92, 216806 (2004).
[3] D. Kitchen et al., Nature 442, 436 (2006).
[4] R. C. Myers et al., Nature Materials 7, 203 (2008).
[5] K. Yang et al., Phys. Rev. Lett. 119, 227206 (2017).
[6] R. E. George et al., Phys. Rev. Lett. 110, 027601 (2013).
Large magnetic field effects, either in conduction or luminescence, have been observed in organic light-emitting diodes (OLEDs) for over a decade now. The physical processes are largely understood when exciton formation and recombination lead to the magnetic field effects. Recently, magnetic field effects in some co-evaporated blends have shown that exciplexes deliver even larger responses. In either case, the magnetic field effects arise from some spin-mixing mechanism and spin-selective processes in either the exciton formation or the exciplex recombination. Precise control of light output is not possible when the spin mixing is either due to hyper-fine fields or differences in the Lande g-factor. We theoretically examine the optical output when a patterned magnetic film is deposited near the OLED. The fringe fields from the magnetic layers supply an additionally source of spin mixing that can be easily controlled. In the absence of other spin mixing mechanisms, the luminescence from exciplexes can be modified by 300%. When other spin-mixing mechanisms are present, fringe fields from remanent magnetic states act as a means to either boost or reduce light emission from those mechanisms. Lastly, we examine the influence of spin decoherence on the optical output.
We investigate the impact of tunnel barrier thickness on electron spin dynamics in Fe/MgO/GaAs heterostructures using spin-resolved optical pump-probe spectroscopy. Comparison of the Larmor frequency between thick and thin MgO barriers reveals a four-fold variation in exchange coupling strength, and investigation of the inhomogeneous dephasing time, T2*, argues that inhomogeneity in the local effective hyperfine field dominates free-carrier spin relaxation across the entire range of barrier thickness. These results provide additional evidence to support the theory of hyperfine-dominated spin relaxation in GaAs at low temperature and in the presence of an externally applied magnetic field. Further, this work lays the foundation for engineering both the exchange coupling and the free carrier spin dynamics in ferromagnet/semiconductor heterostructures, allowing for the exploration of dissipation and transport in the regime of dynamically-driven spin pumping.
Random, spatially uncorrelated nuclear-hyperfine fields in organic materials dramatically affect electronic transport properties such as electrical conductivity, photoconductivity, and electroluminescence. Competition between spin-dynamics due to these spatially uncorrelated fields and an applied magnetic field leads to large magnetoresistance, even at room temperature where the thermodynamic influences of the resulting nuclear and electronic Zeeman splittings are negligible. Here, we discuss a new method of controlling the electrical conductivity of an organic film at room temperature, using the spatially varying magnetic fringe fields of a magnetically unsaturated ferromagnet. Fringe-field magnetoresistance has a magnitude of several percent, and is hysteretic and anisotropic. This new method of control is sensitive to even remanent magnetic states, leading to different conductivity values in the absence of an applied field. The fringe field effects are insensitive to the ferromagnetic film’s thickness (and therefore the fringe field magnitude) but sensitive to the magnetic domain’s correlation length. This points at fringe-field gradients as an important ingredient of this mechanism. We develop a model based on fringe-field induced polaron-pair spin-dynamics that successfully describes several key features of the experimental fringe-field magnetoresistance.
The understanding of spin transport in organics has been challenged by the discovery of large magnetic field effects on properties such as conductivity and electroluminescence in a wide array of organic systems. To explain the large organic magnetoresistance (OMAR) phenomenon, we present and solve a model for magnetoresistance in positionally disordered organic materials using percolation theory. The model describes the effects of singlettriplet spin transitions on hopping transport by considering the role of spin dynamics on an effective density of hopping sites. Faster spin transitions open up `spin-blocked' pathways to become viable conduction channels and hence produce magnetoresistance. We concentrate on spin transitions under the effects of the hyperfine (isotropic and anisotropic), exchange, and dipolar interactions. The magnetoresistance can be found analytically in several regimes and explains several experimental observations
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